Multiparameter flow cytometry, including BrdU content analysis, is being used to determine whether the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) has a role in cell cycle arrest in response to DNA damage. To investigate the effect of the absence of DNA-PKcs on the G1 cell cycle checkpoint, DNA-PKcs +/- and -/- mouse embryonic fibroblasts were irradiated at 6.0 Gy and then continuously labeled with BrdU. By 6 h post-irradiation, cells were depleted from the S phase in both DNA-PKcs +/- and -/- cultures indicating induction of arrest at a G1 checkpoint. Preliminary results show that the extent of G1 arrest was comparable for the DNA-PKcs +/- and -/-cultures. Future studies will involve continuous labeling with BrdU and examination of the cells at longer time intervals following radiation to examine the cell cycle recovery patterns.